The present invention relates to the fields of microelectronics, optics, or optoelectronics, and, more particularly, to bonding semiconductor materials to produce structures for use in the fields of microelectronics, optics, or optoelectronics. Moreover, in certain respects, the invention relates to activating the bonding surface of at least one of two wafers to be bonded that may have been oxidized and/or undergone atomic species implantation.
In applications atomic species implantation can form a zone of weakness within a wafer, termed the “donor” wafer, at a predetermined depth. This method, known as “Smart-Cut” is known to those of ordinary skill in the art. Reference could be made, for example, to the work “Silicon-on-Insulator Technology, Materials to VLSI, 2nd Edition, by Jean-Pierre Colinge, published by Kluwer Academic Publishers, pages 50 and 51). A structure known by the acronym SOI (silicon-on-insulator) can then be produced by implantation through the oxidized surface of a silicon donor wafer and by transferring, by bonding onto a silicon wafer termed the “receiver” wafer, a thin film including the superficial oxide layer and the upper thin layer of silicon derived from said donor wafer.
In today's semiconductor processing industry, the intensifying miniaturization of electronic components formed on such transferred thin films forces substrate fabricators to produce SOI type substrates with ever thinner upper layers of silicon, while seeking to retain excellent quality. As a result, improving the quality of transferred layers and thus improving techniques for taking off and transferring such layers is vital. Bonding quality is essential to good layer transfer, where the quality of that bonding is principally measured by the bonding energy binding the two wafers. As is explained below, it has now been established that the existence of certain contaminants on the wafer surfaces has the effect of reducing the bonding energy.
The implantation step generally brings hydrocarbon type contaminants to the wafer surface. In the presence of isolated particles or localized surface defects, that contamination can result in the formation of superficial blisters following detachment of the thin film and its transfer onto the receiver wafer, or even to the formation of non-transferred zones. Withdrawing such contaminants is thus vital in order to guarantee good quality and bonding contact.
In addition, the article by Aditya Agarwal, T. E. Haynes, V. C. Venezia, O. W. Holland and D. J. Eaglesham entitled “Efficient production of silicon-on-insulator films by co-implantation of He+ with H+”, Applied Physics Letters, vol 72 (1998), pp 1086-1088 shows that while methods employing co-implantation of hydrogen and helium allows a much lower total implantation dose than that for simple implantation to be used, blister type defects are more numerous at the bonding interface.
Further, it has been shown that SOI type substrates with a buried oxide layer that is very thin, i.e. less than 50 nm (50 nanometers) thick, are more difficult to produce, since they are much more sensitive to the appearance of blister type defects. Thus, it is also important to reinforce the bonding energy in order to broaden the working conditions and the application possibilities of said “Smart-Cut” method. Finally, the bonding energy must also be reinforced to encourage proper detachment and layer transfer.
In fact, the existence of contaminants at the bonding interface can result in detachment of the layer at that interface rather than at the zone of weakness, thereby creating defects (non-transferred zones) on the receiver wafer, which correspond to residues on the donor wafer. The lower the bonding energy, the greater the quantity of non-transferred zones. Further, when the bonding energy is low, it is more difficult for the bonding wave to reach the edge of the wafer diametrically opposite to that from which bonding has been initiated, and a larger number of defects is observed in that region.
The prior art already discloses several methods of treating the surface prior to bonding, to improve bonding and to eliminate all particles present on the surface of the wafers to be bonded.
Such treatments generally comprise two successive steps, namely:                a) a first step of cleaning and chemical activation; and        b) a second step of cleaning, which second step is carried out immediately prior to bonding and is termed “clean-before-bond” below.        
The purpose of cleaning the surfaces to be bonded in step a) is to:                render said surfaces hydrophilic;        withdraw contaminants, in particular of the hydrocarbon type, that have appeared on the surfaces of the wafers following implantation;        remove isolated particles;        limit the roughness (on an atomic scale) to ensure that wafers are brought into intimate contact.        
The hydrophilic nature of the surface encourages bonding and can increase the bonding energy, limiting the appearance of defects at the wafer edge.
The prior art contains such a cleaning and activation method, termed “RCA”, which consists in treating the surfaces to be bonded in succession with:                a first solution known as “SC1” (standard clean 1) which comprises a mixture of ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2), and deionized water;        a second solution known as “SC2” (standard clean 2), which comprises a mixture of hydrochloric acid (HCl), hydrogen peroxide (H2O2), and deionized water.        
The first solution is principally intended to withdraw isolated particles present on the wafer surface and to render the surfaces hydrophilic, while the second solution is intended to remove metallic contamination. However, it has been shown that after carrying out such a treatment, the roughness of the wafer surfaces can sometimes be greater than that before the treatment, which considerably alters their bonding energy. Further, French patent application FR 04/0330 discloses a method of cleaning the oxidized surface of a wafer for bonding it to another wafer. Said method employs a mixture of ammonia solution (NH4OH) and hydrogen peroxide (H2O2) and can remove isolated particles while avoiding creating surface roughness. Furthermore, it should be noted that while the above-mentioned Smart-Cut method comprises multiple cleaning steps, the clean-before-bond step b) is highly specific as it predetermines the quality of the substrates obtained after the step for transferring the layer(s). That step is aimed at withdrawing particles that have been deposited during the interval between the cleaning step a) and bonding. It is also aimed at reinforcing the hydrophilic nature of the wafers, as that tends to be reduced substantially as the time interval between the cleaning step a) and bonding is increased.
Cleaning is generally carried out by brushing the surfaces to be bonded with a solution of deionized water; see, for example, U.S. Patent Application 2004-0248379 published Dec. 9, 2004, entitled “Method for Bonding Semiconductor Structures Together” which is hereby expressly incorporated herein by reference.
With the above-mentioned two-step method, it unfortunately appears that the more the surface to be bonded is rendered hydrophilic in order to limit the number of transfer defects, the more the roughness of the surface increases, thus increasing the probability of the appearance of blister type defects.
In the above-mentioned RCA type treatment, a greater hydrophilic nature is obtained by using the SC 1 solution at a high temperature (>70° C.). However, in contrast, the treated surface will be etched, which will increase its roughness, and that increase in surface roughness increases with increasing temperature of the SC 1 bath.
Finally, an additional constraint also appears when using the above-mentioned method, as the time interval between the two steps must be minimized in order to preserve the hydrophilic nature of the treated wafers and to maximize the bonding energy. When carrying out the fabrication method on an industrial scale, this constraint results in additional constraints when managing batches of wafers to be treated.